Estimating permeability of sedimentary formations is one of the most important factors in distinguishing economic from uneconomic reservoirs. Generally, however, the estimation of permeability from log data has been only partially successful. The Coates-Timur relationship is widely used in magnetic resonance well logging to correlate permeability to two parameters, porosity (φ) and bound fluid volume (BFV) as follows:                     k        =                  a          ⁢                                          ⁢                                                    ϕ                b                            ⁡                              (                                                      ϕ                    -                    BFV                                    BFV                                )                                      c                                              (        1        )            where a, b, and c are empirical constants with common values of 10000, 4, and 2. BFV is the product of porosity and irreducible water saturation, Swirr, so the equation above can also be given as:                     k        =                  a          ⁢                                          ⁢                                                    ϕ                b                            ⁡                              (                                                      1                    -                                          S                      wirr                                                                            S                    wirr                                                  )                                      c                                              (        2        )            Using the default values of a, b, and c, this can be rearranged to:                               S          wirr                =                              100            ⁢                                                  ⁢                          ϕ              2                                                          100              ⁢                                                          ⁢                              ϕ                2                                      +                          k                                                          (        3        )            
Substantially more detailed discussions regarding the Coates-Timur equation can be found in:    Timur, A., 1969, “Producible porosity and permeability of sandstones investigated through NMR principles,” Log Analyst, 10(1), 3–11;    Ahmed, U., Crary, S. F. and Coates, G. R., 1989, “Permeability estimation: the various sources and their interrelationship,” SPE 19604; and    Coates, G. R., Miller, M., Gillen, M. and Henderson, G., 1991, “The MRIL in Conoco 33-1—an investigation of a new magnetic resonance imaging log,” Trans. SPWLA 32th Ann. Log. Symp., New Orleans, La., Paper DD.Each of these references are incorporated by reference herein in their entireties.
However, the Coates-Timur relationship between porosity, irreducible water saturation and permeability often does not adequately describe carbonate formations, which account for approximately 60% of the earth's hydrocarbon reserves.
Accordingly, it is one object of the present invention to present a method of correlating porosity, bound fluid volume, and permeability for most types of carbonate rocks.